1. Field of the Invention
The present invention relates generally to the production of shaped, tempered sheets of glass and, more particularly, to an improved method of and apparatus for shaping and heat treating relatively thin glass sheets.
Shaped glass sheets are widely used as side windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows are installed. It is also important that the side windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. To promote efficient and large scale production, discrete glass sheets are conventionally heated, bent and cooled while being moved continuously along a fixed path and successively through a heating section, a roll forming section, a quenching section and a cooling section. To achieve satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level so as to maintain the core or interior thereof above a deformation temperature upon being exposed initially to the quenching medium at the quenching section. The residual heat remaining in glass sheets of previous commercial thicknesses, such as those having nominal thicknesses ranging from 4.5 millimeters to 6 millimeters, is generally sufficient after shaping for immediate advancement to the tempering area and exposure to the quenching medium. Thus, the heat initially imparted to a relatively thick glass sheet to bring it to proper temperature for shaping can also be utilized in the final heat treating operation.
However, within the last several years, considerable emphasis has been placed on the use of thinner and thinner glass sheets for automobile side windows as a means of reducing overall weight of the autos as a means to obtain better fuel mileage. This has posed problems in shaping and tempering, due to the lesser ability of the thinner sheets to retain heat and the aforementioned conventional process of bending and treating glass sheets does not lend itself to the processing of these relatively thin sheets, such as those having nominal thicknesses ranging from less than 3 millimeters to 4 millimeters (90 mils to 160 mils). As the thickness of the glass decreases, the rate of heat loss increases and the heat initially imparted to such thin sheets is quickly dissipated upon leaving the heating atmosphere of the furnace and during the relatively cool bending cycle. Attempts to solve these problems by initially overheating the thin glass sheets have not been successful because of the consequent loss of control of the glass shaping process and the degradation of the surface quality of the finished glass as a result of heat stains, roll ripple distortion, and the imposition of roll marks in the surface of the heat-softened glass sheet.
Consequently, roll forming has been developed as a technique for shaping and tempering glass sheets at a high production rate. One of the benefits of the roll forming process is the rapid removal of each individual glass sheet from the heating section or furnace through the shaping section and into the quenching section. In the roll forming method, glass sheets are conveyed without stopping through heating, shaping, and tempering sections along high speed glass sheet conveyor means to drastically reduce the time needed to traverse the distance between the exit of the heating section or furnace to the tempering or quenching section to a minimum, preferably under 5 seconds. Under such circumstances, thin glass sheets can be tempered by quenching without imparting such a high initial temperature at the furnace that shape control and control of surface quality is lost as a consequence of insuring that the temperature at the core of each glass sheet does not cool to below the minimum temperature needed on arrival at the quenching section to assure adequate temper.
Quenching or tempering medium is applied against the opposite major surfaces of the shaped glass sheets. In the past, a movable gate was sometimes provided to minimize back flow of quenching medium into the shaping section. This involved the inclusion of a moving element whose movement must be correlated with the movement of individual glass sheets from a shaping section to a quenching section.
In rool forming as practiced in the prior art, either a continuous glass ribbon or a series of discrete glass sheets is heated to or above the deformation temperature of the glass and passed in a continuous motion through one or more shaping stations where the shape of the glass is changed from a flat configuration to a shaped configuration. Shaping individual glass sheets by roll forming, particularly those of non-rectangular shape having one or both longitudinal side edges extending obliquely of the path of glass sheet movement, is more difficult to perform than roll forming a continuous ribbon, because individual glass sheets have leading edges as well as side edges that are prone to be distorted by a high speed shaping operation, whereas only the side edges of a continuous ribbon are more prone to distortion than the main body of the glass.
Glass sheets have been warped or distorted into different configurations, that is, from flat to curved or from curved to flat by either differentially heating or differentially cooling the opposite glass sheet surfaces. Shaped glass sheets have been subjected to a slight pressure differential to maintain the shaped glass sheets in frictional engagement with shaped rotating conveyor rolls that propel shaped glass sheets through a quenching section where chilling medium is applied to the heated shaped glass sheets rapidly enough to impart a temper thereto. However, thin glass sheets distorted solely by differential heating and/or differential cooling have been known to develop an "oil canning" effect in which the thin distorted glass sheet flexes uncontrollably between metastable states of opposite flexure compared to a flat sheet.
The history of prior art attempts to shape glass sheets continuously without causing the glass sheets to stop for the shaping step so as to obtain as high a production rate of shaped glass sheets as possible and the problems associated with shaping thin glass sheets by differential heating and/or differential cooling will be understood better in the light of a description of the prior art that follows.
2. Description of the Prior Art
Many patents have been issued on roll forming.
Drake U.S. Pat. No. 2,348,887 moves heated glass sheets between a pair of aligned pressure rolls 32 and 33 of cylindrical configuration which force the bottom surfaces of the glass sheets to ride over a series of spaced bending rolls 31 of cylindrical configuration mounted for rotation along spaced lines that extend transversely of a curved path corresponding to the shape desired for the bent glass sheets. The shapes imparted to the moving glass sheets are limited to cylindrical curvatures of uniform radius about an axis transverse to the path of glass movement.
Jamnik U.S. Pat. Nos. 3,226,219 and 3,284,182 and Jamnik and Pelzl U.S. Pat. Nos. 3,245,771 and 3,248,198 form a continuous ribbon of glass into cross-sectional contours of U-shaped configuration by passing the ribbon between consecutive pairs of rolls comprising complementary upper and lower forming rolls of gradually increasing severity of shape. These patents shape continuous ribbons of glass rather than discrete glass sheets.
Humphreys U.S. Pat. No. 3,420,650 forms a continuous ribbon of U-shaped configuration by first tensioning the flat ribbon to adjust its width while hot and then shaping the hot, tensioned ribbon to a U-shaped contour. This patent treats a continuous ribbon rather than discrete glass sheets.
Bogrets U.S. Pat. No. 3,820,969 moves forming elements toward one another to make profiled articles from a ribbon of hot moving glass. The glass is shaped relative to an axis extending along the path of glass movement. The movement of a forming element must be correlated with the movement of the other forming element and with the glass movement for this system to operate effectively.
Ritter et al. U.S. Pat. No. 3,881,906 sags heated glass sheets to intermediate shapes of progressively increasing curvature transverse to their path of movement by conveying said heated glass sheets on successive, contoured, rotating, conveyor rolls of increasing transverse curvature en route to a shaping station. The entire weight of a transversely extending leading element of the glass is borne entirely along the side edge portions of the glass as it transfers from one contoured forming roll to the next. Consequently, the lateral edges kink away from the overall curvature desired and it is necessary to stop each partially shaped glass sheet at a shaping station where its shaping is completed by the inertia gravity method which involves the use of a shaping mold that moves in an upward vertical direction transverse to both the glass movement path and the axes of rotation of the contoured rolls to engage the glass sheet margin while the glass sheet forward movement is stopped. This patent also provides a moving gate between the shaping station and the quenching station to limit back flow of quenching medium from the quenching station to the shaping station. Therefore, this patented apparatus must coordinate the movement of a shaping mold and a gate with the glass sheet movement.
Nedelec U.S. Pat. No. 3,545,951, Bezombes U.S. Pat. No. 3,801,298 and U.S. Pat. No. 3,832,153 and Hoff et al. U.S. Pat. No. 3,831,239 shape moving glass sheets between shaped conveyor rolls that support the lower surface of moving heat-softened glass sheets and a movable upper shaping member of complementary configuration. The apparatus of these patents provides a family of simple curves about a single axis transverse to the path of glass sheet movement. These patents require the shaped conveyor rolls to rotate between different orientations from a flat glass supporting position to a shaped glass supporting position. The change in orientations must be correlated with glass sheet movement to obtain desired results.
Frank U.S. Pat. Nos. 3,701,644; 3,856,499; 3,871,855; 3,891,420; 3,929,441; 3,934,996; 3,992,181 and 4,043,783, and Knapp U.S. Pat. No. 3,869,269 disclose roll forming apparatus capable of shaping a succession of discrete moving glass sheets to either simple shapes provided with one component of shape about either an axis extending longitudinally of the path of glass sheet movement or about an axis extending transversely thereof or compound shapes involving various combinations of two components conforming to said simple shapes. In addition, the roll forming apparatus of this group of patents is capable of shaping glass sheets to either simple or compound shapes involving non-uniform radii of curvature.
This last group of patents provides different inventions incorporated in the most sophisticated system for shaping continuously moving glass sheets to various shapes at the highest rates of production attained prior to the present invention. However, even though this last group of patents provided highest production rates and the greatest variety of simple and compound shapes for glass sheets ever attained, the apparatus comprised movable parts whose movement between spaced apart positions on opposite sides of a path of movement provided by conveyor rolls for glass sheets and glass engaging positions to one side of said conveyor rolls had to be correlated with the glass sheet movement between the movable parts. This correlation required constant monitoring and frequent adjustment of moving parts. In addition, it was necessary to spend considerable time for set up and adjustment of the apparatus when production patterns were changed to insure that the movements of the rotating shaping rolls toward and away from one another correlate properly with the movement of discrete glass sheets therebetween.
Many patents have also issued on thermal warpage of treated glass sheets. These patents use differential heating or cooling or a combination of differential heating and differential cooling against the opposite surfaces of the glass sheet to shape the glass to a different shape from its original shape.
U.S. Pat. No. 3,223,499 to Cypher and Davidson differentially heats the glass sheet while conveyed on a roller hearth to induce an upward warp, then the heat differential is reduced to reduce the warp while continuing to heat the sheet. The heated sheet may be supported on a roller hearth or a gas hearth.
U.S. Pat. No. 3,245,772 to Cypher and Davidson covers thermal warping by differential heating while conveying glass sheets on a roller conveyor extending through a furnace.
U.S. Pat. No. 3,262,768 to Carson temporarily warps a selected edge portion of glass sheet away from an outline mold to which it has been shaped by gravity sagging by differentially applying cooling fluid against the opposite glass sheet surfaces so as to ensure better cooling of the warped edge portion of the shaped glass sheet supported on the outline mold for bending.
U.S. Pat. No. 3,332,761 to Fredley and Sleighter discloses the application of cold air upward at a rate sufficient to provide glass sheet support while annealing glass sheets in spaced relation over a gas hearth.
U.S. Pat. No. 3,342,573 to Fredley and Sleighter discloses supplying a support gas at different pressures at different parts of a gas hearth.
U.S. Pat. No. 3,372,016 to Rahrig, O'Connell and Ferguson discloses differentially heating a glass sheet to bow the sheet upward and then heating from below only to tend to remove the warp that is formed by the initial differential heating.
U.S. Pat. No. 3,396,000 to Carson, Ferguson, Ritter and Hymore discloses quenching opposite surfaces of the glass sheet at preselected different rates to warp a flat sheet to a desired curvature.
U.S. Pat. No. 3,497,340 to Dennison and Rigby discloses a differential rapid cooling of opposite sides of glass sheets through the tempering temperature range to cool the faster cooling side through the temperature tempering range then reducing the faster cooling rate to maintain that side cooled at a lesser cooling rate at a temperature high enough to maintain the glass sheets at a first configuration and then continuing cooling until the sheets are no longer deformable through viscous flow whereby a second configuration forms in the glass sheet.
U.S. Pat. No. 3,522,029 to Carson and Ritter discloses shaping glass sheets by differentially cooling one surface from the central area to an edge area and also shaping glass sheets by differential cooling of the opposite surfaces during movement along a multiple speed conveyor.
U.S. Pat. No. 4,028,086 to Rahrig and Revells discloses passing glass sheets through a quench area where a pressure differential between the top and bottom surfaces is applied to force the glass sheet upward against upper conveyor rolls and to warp or shape the sheet by cooling its bottom surface faster than its top surface.
None of the patents disclose shaping a glass sheet by roll forming to one configuration and changing the configuration by differential cooling.